Apex Triggers - 17 (Deloitte Interview Question)

 1. Scenario Overview

This automation solves a Deloitte Interview Question focused on enforcing data integrity across parent-child relationships . The contact object features a custom checkbox field, Primary_Contact__c . The requirement mandates that each Account can have at most one primary contact . If a user tries to create or update a contact to be a duplicate primary contact under the same account—or re-parents an existing primary contact to an account that already contains one—the transaction must block and throw an error . 

Because data validation must block execution before commit phases, these criteria execute entirely in before insert and before update context steps .

or

Enforce a single Primary Contact on an Account.

---

2. Apex Handler Class

java

public class ContactTriggerHandler {
    public static void preventDuplicatePrimaryContacts(List<Contact> newContacts, Map<Id, Contact> oldContactMap) {
        Set<Id> accountIds = new Set<Id>();
        
        // Collect targeted parent account IDs across insert, update, and re-parenting events
        if (newContacts != null) {
            for (Contact con : newContacts) {
                if (con.AccountId != null) {
                    if (oldContactMap == null) {
                        // Insert Context: Track account if contact is marked primary
                        if (con.Primary_Contact__c == true) {
                            accountIds.add(con.AccountId);
                        }
                    } else {
                        // Update Context: Track accounts for changes in primary status or parent accounts
                        Contact oldCon = oldContactMap.get(con.Id);
                        if (con.Primary_Contact__c == true || oldCon.Primary_Contact__c == true) {
                            accountIds.add(con.AccountId);
                            if (oldCon.AccountId != null && con.AccountId != oldCon.AccountId) {
                                accountIds.add(oldCon.AccountId); // Capture old parent if re-parented
                            }
                        }
                    }
                }
            }
        }
        
        if (accountIds.isEmpty()) {
            return;
        }
        
        // Map out the number of pre-existing primary contacts for each tracked Account
        Map<Id, Integer> accountPrimaryCountMap = new Map<Id, Integer>();
        for (Contact existingCon : [SELECT Id, AccountId FROM Contact 
                                    WHERE AccountId IN :accountIds 
                                    AND Primary_Contact__c = true]) {
            if (accountPrimaryCountMap.containsKey(existingCon.AccountId)) {
                accountPrimaryCountMap.put(existingCon.AccountId, accountPrimaryCountMap.get(existingCon.AccountId) + 1);
            } else {
                accountPrimaryCountMap.put(existingCon.AccountId, 1);
            }
        }
        
        // Validate records against criteria and add DML-blocking form errors
        for (Contact con : newContacts) {
            if (con.AccountId != null && con.Primary_Contact__c == true) {
                Integer existingCount = accountPrimaryCountMap.containsKey(con.AccountId) ? accountPrimaryCountMap.get(con.AccountId) : 0;
                
                if (oldContactMap == null && existingCount > 0) {
                    // Fail if inserting a new primary contact where one exists
                    con.addError('This account cannot have more than one primary contact.');
                } else if (oldContactMap != null) {
                    Contact oldCon = oldContactMap.get(con.Id);
                    
                    // Case A: Contact was already primary and re-parented to an account that has one
                    // Case B: Contact changed from regular to primary on an account that already has one
                    if ((oldCon.AccountId != con.AccountId && existingCount > 0) || 
                        (oldCon.AccountId == con.AccountId && oldCon.Primary_Contact__c == false && existingCount > 0)) {
                        con.addError('This account cannot have more than one primary contact.');
                    }
                }
            }
        }
    }
}

Use code with caution.

---

3. Apex Trigger

java

trigger ContactTrigger on Contact (before insert, before update) {
    // Lock validation executions safely inside 'before' contexts to capture errors prior to commitments
    if (Trigger.isBefore) {
        if (Trigger.isInsert) {
            ContactTriggerHandler.preventDuplicatePrimaryContacts(Trigger.new, null);
        } else if (Trigger.isUpdate) {
            ContactTriggerHandler.preventDuplicatePrimaryContacts(Trigger.new, Trigger.oldMap);
        }
    }
}